The invention is directed to a special cross section of the supporting edge of soft contact lenses or hard soft contact lenses (hard in the center, soft at the edge), as used for correction of visual defects.
With soft or hard soft lenses, it is necessary for the overall diameter of the lenses to be greater than the corneal diameter, that is to say, always greater than 11.0 mm., preferrably approximately 15.0 mm., whereby the diameter of the optical zone necessary for correction of the visual defect is only approximately 8.0 mm. The zone between the outer diameter of the optical zone and the outer edge of the lens is termed the supporting edge which configuration must be specially matched to the shape of the eyeball. Particularly in this area, there are innumerable curves on the eyeball which are not measurable or hardly measurable. In addition, these curves are different with every eye; at the same time, the diameter of the cornea is always different as well. The cornea also protrudes from the eyeball by a very pronounced curve. The limbus is located in this area and is the tissue between the cornea and the sclera of the eyeball. If careful accurate matching of the supporting edge of the lens to the eye profile is not achieved, significant problems can frequently be expected. One of these problems is that the soft lens deforms and thus provides reduced visual acuity. Also, it is possible for pressure, abrasion, reddening, inflammation and deficient metabolism to occur as a result of constricted or partially constricted lacrimal film exchange underneath the lens due to inaccurate matching. This means reduced subjective and and objective wearing comfort.
There are various supporting edge cross sections being used in soft contact lenses to attempt to overcome these problems. For example, one such known cross section diminishes in a wedge-shape fashion from the edge of the optical zone to the lens edge or in another design diminishes very sharply from the optical edge and then continues parallel past the whole supporting edge section up to the lens edge. The supporting edge is in the latter case intentionally cut thin, preferably 0.10 mm. and automatically provides individual conforming curves when in contact with the eye.
In the case of the cross section which diminishes in a wedge-shape fashion from the optical edge up to the edge, optimum automatic fitting is only possible at the edge of the lens. At this point, the edge, however, should be somewhat thickened to provide ideal tear resistance. At the limbus on the other hand, or in the central area (diameter range of approximately 11.5 mm.), the lens is at its thickest or almost at its thickest. It is here, however, that the lens should have optimum conformability; that is to say, not intentionally thick, but it should be as thin as possible to obtain maximum automatic conformability to the eye profile. It is only in this manner that the use of the lens and its range of application can be optimized. Such wedge-shape diminution toward the edge of the cross section of the lens is independent of the number of internal curves of the lens.
In connection with the aforementioned sharp diminishment of the cross section of known lenses, the cross section is linked to a single curve form of the inner surface of the lens, the supporting edge of which runs parallel from the edge past the limbus, and then merges as an oblique curve into the thicker central optical area. When in contact with the eye, this supporting edge can automatically conform individually to the curves of the eye through the principle of the kinked curves. The thinner the supporting edge, the greater this characteristic. However, the tear resistance is now reduced in the case of the lens edge which is now thinner. The instability of the lens and difficulty of handling increase at the same time, so that a supporting edge thickness of less than 9/100 mm. is not recommended. This factor is also supported by the difficulty of manufacture. If, on the other hand, the supporting edge exhibits a thickness greater than 12/100 mm. standard, the same problems regarding conformability reoccur as in the case of the cross section of the above mentioned soft lens.